Telecommunication systems where multiple users communicate with a common communication station are well known in the art. Such systems are defined by the specifications for the Third Generation Partnership Project (3GPP). In a 3GPP system, a number of user equipment (UEs) can be camped on a system cell for communication with a Radio Network Controller (RNC) via various channels including an uplink Random Access Channel (RACH). For example. FIG. 5 illustrates a typical 3GPP system called a Universal Mobile Telecommunications Systems (UMTS).
As shown if FIG. 5. a typical UMTS system architecture includes a Core Network (CN) interconnected with a UMTS Terrestrial Radio Access Network (UTRAN) via an interface known as Iu which is defined in detail in the publicly available 3GPP specification documents. The UTRAN is configured to provide wireless telecommunication services to users through so-called User Equipments (UEs) in 3GPP. via a radio interface known as Uu. The UTRAN has base stations, known as Node Bs in 3GPP, which collectively provide for the geographic coverage for wireless communications with UEs. In the UTRAN, groups of one or more Node Bs are connected to a Radio Network Controller (RNC) via an interface known as Iub in 3GPP. The UTRAN may have several groups of Node Bs connected to different RNCs, two are shown in the examvle depicted in FIG. 5. Where more than one RNC is provided in a UTRAN, inter-RNC communication is performed via an Iur interface having two RNCs in a UTRAN, inter-RNC communication is performed via an Iur interface.
Wireless communication with UEs in a 3GPP system is conventionally done utilizing the transmission of successive radio frames divided into timeslots as illustrated in FIG. 6. The system may be configured with commonly used time slots (CUTSs) such as illustrated in FIG. 7, preferably one such time slot for each system frame, which are commonly available to the UEs for Random Access Channel (RACH) transmission. As shown in FIG. 7, a typical CUT is configured with a midamble sandwiched between two data portions with a Guard Period (GP) provided for facilitating timing adjustments A UE may attempt a RACH transmission and select a RACH CUTS of a random frame using one of N code identifiers, for example, one of eight midambles. If no other UE transmits in the same slot with the same midamble and if there is sufficient Signal to Noise Ratio (SNR), then the UE's RACH transmission succeeds. If another UE transmits in the same slot with the same midamble, then they both fail. If another UE transmits in the same time slot with a different midamble, then they both succeed provided that they each have sufficient SNR.
If a UE transmits in a time slot and there are N other UEs also transmitting in that slot, assuming that there is more than adequate SNR, then the probability of a UE succeeding is P=(1−1/M)N, where M is the number of code identifiers (e.g. eight (8) midambles for a preferred 3GPP system) and N is the number of other UEs transmitting. The average number of successes per time slot is:Neff=N(1−1/M)N  Equation 1
This can be extended to consider rates of accesses per second. For example, from a UE's perspective, if it can transmit on a RACH channel characterized by 8 codes per slot and one slot per frame, there are 800 resources available per second for 3GPP standard 10 microsecond frames. Then N may be the average number of accesses per second and M may be the number of resources, i.e. 800 in the example. As M gets large, a frequently used approximation is valid:P=exp(−N/M)  Equation 2
It is well known that under these assumptions, the maximum success rate occurs when N=M and this rate is Pmax=M/e. However, there is a cost for operating at this level. The average UE will experience several failures before it succeeds and time delay becomes an issue. The time delay is composed of the time it takes to for the UE to identify that it has not succeeded plus the time for a retry, and this delay may occur more than once (i.e. there may be several failures).
In 3GPPP there is a relatively long layer three (3) acknowledgment time, on the order of seconds, so that the recommended operating condition for the RACH, at least the basic access channel, is preferably biased towards having very few collisions. Preferably, the system will operate in the neighborhood of no more than one access attempt per RACH time slot. For a standard 3GPP system frame of 10 microseconds, this translates to a rate of 100 accesses per second.
In 3GPP, TS 25.331, a parameter of dynamic persistence (DP) is defined which is set by the Radio Network Controller (RNC) to avoid saturation of the Random Access Channel. The RNC broadcasts DP or a DP level to the UEs and the UEs adjust their rate of access to the RACH time slots as a function of DP. The inventor has recognized that the rate of access can be changed to produce more efficient communication by adjusting DP based upon the number of successful and failed RACH transmissions.
The inventor has also recognized that insufficient SNR which causes failed RACH transmissions can be the result of the UEs transmitting with insufficient power. In 3GPP, TS 25.331, a RACH Constant parameter is defined which is broadcast by the RNC and is used by the UEs to determine the power of RACH transmissions. The inventor has recognized that more efficient communications can result by adjusting the RACH Constant based upon successes and failures of RACH transmissions.